1. Field of the Invention
The present invention relates to a communication system. More particularly, the present invention relates to an apparatus and method for allocating frequency resources in a communication system.
2. Description of the Related Art
Next generation communication systems are evolving with an emphasis on providing mobile stations (MSs) with services capable of transmitting/receiving large amounts of data at high speed.
One type of a next generation communication system is a communication system having a cellular structure (hereinafter, referred to as a “cellular communication system”). Cellular communication systems include a plurality of cells to which limited resources, such as frequency resources, code resources, and time slot resources, are distributed. The use of the limited resources in the cells may result in inter-cell interference (ICI). Two conventional methods for canceling the ICI include an interference cancellation scheme and a cell planning scheme.
In the interference cancellation scheme ICI is canceled after the ICI has occurred. Implementation of the interference cancellation scheme in a cellular communication system is difficult due to its complexity and the need for information, such as channel information of adjacent cells. In contrast, the cell planning scheme is comparatively easier to implement. In the cell planning scheme frequency resources are used in cells according to a plan that was made in advance to prevent ICI from occurring. As a result of the cell planning scheme being easier to implement, cellular communication systems tend to use the cell planning scheme.
The cell planning scheme includes a reuse partitioning (RP) scheme and a soft frequency reuse (SFR) scheme.
An example of resources being allocated using the reuse partitioning scheme in a conventional communication system will be described with reference to FIG. 1.
Referring to FIG. 1, according to the reuse partitioning scheme, when a communication system includes three cells, i.e. cell A 110, cell B 120, and cell C 130, the cells 110, 120 and 130 are divided into first regions 111, 121 and 131 and second regions 113, 123 and 133, respectively, according to distances from the respective base stations. The first regions 111, 121 and 131 correspond to centers of the cells, and the second regions 113, 123 and 133 correspond to edges of the cells. MSs located in the first regions 111, 121 and 131 are allocated the same frequency band, i.e. band #1, and MSs located in the second regions 113, 123 and 133 are allocated mutually different frequency bands, i.e. band #2, band #3 and band #4, respectively, depending on the cell.
Accordingly, since the MSs located in the second regions 113, 123 and 133 encounter a stronger ICI, i.e. experience a smaller signal-to-interference-plus-noise ratio (SINR), as compared to the MSs located in the first regions 111, 121 and 131, the MSs located in the second regions are allocated mutually different frequency bands, thereby reducing the strength of the ICI in the second regions. In addition, although the MSs located in first regions 111, 121 and 131 encounter a weaker ICI, i.e. experience a larger SINR, as compared to the MSs located in the second regions, the MSs located in the first regions are allocated the same frequency band, thereby reducing the strength of the ICI in the first regions on account of power attenuation being dependent on distance. However, according to the reuse partitioning scheme, since cell A 110 is allocated bands #1 and #2 among all of the frequency bands, cell B 120 is allocated bands #1 and #3, and cell C 130 is allocated bands #1 and #4, efficiency is low in terms of resource utilization.
FIG. 2 is a view illustrating an example of resources being allocated according to the soft frequency reuse scheme in a conventional communication system.
According to the soft frequency reuse scheme, when a communication system includes three cells, i.e. cell A 210, cell B 220, and cell C 230, the cells 210, 220 and 230 are divided into first regions 211, 221 and 231 and second regions 213, 223 and 233, respectively, according to distances from the respective base stations. The first regions 211, 221 and 231 correspond to the centers of the cells, and the second regions 213, 223 and 233 correspond to edges of the cells. MSs located in the second regions 213, 223 and 233 are allocated mutually different frequency bands, i.e. band #1, band #2 and band #3, and relatively high power, and MSs located in each first region 211, 221 or 231 are allocated the same frequency band as those allocated to MSs located in the second regions 221 and 231, 231 and 211, or 213 and 223 of cells other than each corresponding cell, and relatively low power.
Accordingly, since the MSs located in the second regions 213, 223 and 233 encounter a stronger ICI, i.e. experience a smaller SINR, as compared to the MSs located in the first regions 211, 221 and 231, the MSs located in the second regions are allocated mutually different frequency bands, thereby the ICI becomes weaker in the second regions. In addition, although the MSs located in the first region 211, 221 and 231 of each cell encounter a weaker ICI, i.e. experience of larger SINR, as compared to the MSs located in the second regions 213, 223 and 233, the MSs located in the first regions are overlappingly allocated the same frequency bands as those pre-allocated to MSs located in the second regions of other cells, thereby the ICI becomes weaker in the first regions on account of power attenuation being dependent on distance.
Therefore, MSs located in cell A 210 can use band #1 in the second region 213 and bands #2 and #3 in the first region 211, MSs located in cell B 220 can use band #2 in the second region 223 and bands #1 and #3 in the first region 221, and MSs located in cell C 230 can use band #3 in the second region 233 and bands #1 and #2 in the first region 231. Thereby, the soft frequency reuse scheme provides a higher resource utilization than the reuse partitioning scheme. FIG. 2 illustrates the resource allocation operation for cell A 210 as an example, excluding the resource allocation operations for cell B 220 and cell C 230.
Meanwhile, a capacity for frequency use in a conventional communication system may be defined by Equation 1 below.
                              W          ⁢                                          ⁢                                    log              2                        ⁡                          (                              1                +                                  S                  ⁢                                                                          ⁢                  I                  ⁢                                                                          ⁢                  N                  ⁢                                                                          ⁢                  R                                            )                                      ≈                  (                                                                                          (                                          W                      *                      S                      ⁢                                                                                          ⁢                      I                      ⁢                                                                                          ⁢                      N                      ⁢                                                                                          ⁢                      R                                        )                                    ⁢                                      log                    2                                    ⁢                  e                                                                              (                                                            S                      ⁢                                                                                          ⁢                      I                      ⁢                                                                                          ⁢                      N                      ⁢                                                                                          ⁢                      R                                        →                    0                                    )                                                                                                      W                  ⁢                                                                          ⁢                                      log                    2                                    ⁢                  S                  ⁢                                                                          ⁢                  I                  ⁢                                                                          ⁢                  N                  ⁢                                                                          ⁢                  R                                                                                                                                                (                                                  S                          ⁢                                                                                                          ⁢                          I                          ⁢                                                                                                          ⁢                          N                          ⁢                                                                                                          ⁢                          R                                                〉                                            〉                                        ⁢                                                                                  ⁢                    1                                    )                                                              )                                    (        1        )            
In Equation 1, “W” represents a bandwidth, and “SINR” represents a signal-to-interference-plus-noise ratio.
That is, the SINR exerts a great effect on capacity when the SINR is a small value, and the “W” exerts a great effect on capacity when SINR is a large value. Therefore, the soft frequency reuse scheme, where MSs located in the second regions of neighboring cells are overlappingly allocated pre-allocated frequency bands as expanded frequency bands, is more efficient in terms of capacity, as compared to the reuse partitioning scheme, where MSs located in the first region experience a relatively larger SINR than the second regions are allocated mutually different frequency bands.
The capacities resulting from the reuse partitioning scheme and soft frequency reuse scheme will now be comparatively described with reference to a graph illustrated in FIG. 3.
The first region of a cell has the highest capacity when the soft frequency reuse scheme is used. The soft frequency reuse scheme makes it possible to obtain a high gain in terms of bandwidth use efficiency despite having a poor performance in terms of SINR. Thus, as compared to the reuse partitioning scheme, the soft frequency reuse scheme exhibits superior performance in terms of capacity. Meanwhile, in the second region of a cell, despite the capacity obtained by the soft frequency reuse scheme being lower than that obtained by the reuse partitioning scheme at values below a preset noise level, a difference between the capacities is not substantial. Therefore, as a whole, a cell has higher performance in terms of capacity when the soft frequency reuse scheme is used.
As described above, using the soft frequency reuse scheme achieves a higher performance than using the soft frequency reuse scheme, in terms of capacity. However, in this case, since MSs located in the first region and MSs located in the second region within the same cell are allocated mutually different frequency bands, bandwidth use efficiency is low.